This page takes one picture from your webcam every second and reads three angles off it: how far your head sits in front of your shoulders, how far your shoulder line is off level, and how far your head is tilted sideways. The first one matters most - a neck carried too far forward is the reason this page exists. If any of them goes past your limit, it makes a noise, on every single crooked reading. The computing happens here in the browser: a pose model finds 33 points on your body and the page works out the angles itself. No key, no quota, and no picture leaving this device - the readings stay in this browser too.
Allow the camera and leave the page open. There is nothing to set up and nothing to calibrate: all three angles are ratios measured on your own body, so they do not depend on how tall you are and read the same for anyone. The page measures from the very first frame and keeps the screen awake while it checks.
The page reads one picture a second, always. Because everything is computed locally, that costs nothing but a little battery - there are no requests to count.
Eight to choose from: parking sensor, fart, Geiger counter, ahem, scream, knuckle crack, whip crack and rimshot. The parking sensor is the default. It starts beeping slowly as soon as you begin to slump, beeps faster the further off you sit, and from twice the limit turns into one solid tone, the way a car sounds just before the wall. It falls silent the moment you sit up. The sounds ship inside the page, and nothing is downloaded. The parking sensor, the fart and the Geiger counter are free; the others are unlocked with the Shrimpcoin you earn by sitting straight.
How far your head juts forward is estimated along depth - the least certain axis a single camera has. So a snapshot is soft and the trend is not. The bar strip shows the day, the curve below shows the days; three red bars in a row mean something, one red bar between green ones is a moment. Pictures with nobody in them do not count at all.
Whether anything is changing does not show in one afternoon. At one picture a second a working day is tens of thousands of readings, so the individual ones are cleared after two days - the daily row from them is kept forever. The comparison „last 7 days against the 7 before“ is the real answer to „am I getting better“.